1. Field of the Invention
This invention relates in general to devices for measuring threads of fasteners, and in particular to a device for measuring internal threads of fasteners.
2. Description of the Prior Art
It is important in many industries to assure that internally threaded fasteners, such as nuts, have the proper thread dimensions. Internal thread gages have been used to measure the functional and pitch diameters of the threads. Pitch diameter is the diameter of an imaginary cylinder, the surface of which would pass through the thread profiles at such points as to make the width of the thread groove (measured parallel to the axis) equal to one-half of the basic pitch. Functional diameter is the pitch diameter of an enveloping thread with perfect pitch, lead, and flank angles and having a specified length of engagement. This diameter includes the cumulative effect of variations in lead (pitch), flank angle, taper, straightness, and roundness. Variations at the thread crest and root are excluded. Many specifications require that both of these measurements be made.
A typical internal thread gage has a frame with two mounting blocks. One of the mounting blocks will move along the frame axis relative to the other. A gage finger will mount to each mounting block. If the gage finger is of a type for measuring functional diameter, it will have a plurality of semi-cylindrical threads. These mate with threads formed on the other gage finger.
By moving the movable mounting block, the diameter of the two gage fingers can be constricted. This allows the user to insert a nut over the threads. The user then allows the movable mounting block to move in the opposite direction, normally urged by springs. The threads of the gage fingers will engage the threads of the fastener. A meter measures the amount of travel of the movable block from a zero position to provide a reading of the functional diameter.
One problem With this type of gage is that the threads of the gage fingers may not fully mesh with the threads of the fastener. The two gage fingers are formed by carefully machining threads on a single shaft, then cutting the shaft in half along the axis of the shaft. The helical path of the threads of the two halves will match only when the gage fingers are positioned precisely at the original dimension of the shaft. This dimension may not be precisely the dimensions of the fasteners being measured. Mismatch of the helical path will result in the gage fingers not fully meshing their threads with that of the fastener. This may be undetected by the user and result in an erroneous reading.
Another problem with prior art techniques is in having to measure both functional diameter and pitch diameter. Pitch diameter must be measured by gage fingers that have only a single thread or protuberance, called a "cone", on one gage finger. The other gage finger will have only two threads or protuberances, called a "vee". The cone and vee are formed on separate gage fingers from the gage fingers that have functional diameter threads. Consequently, the user must have a separate frame and meter for pitch diameter measurements, or he must mount and remove the two different types of gage fingers from a single frame.